The present invention relates to processes for controlling the deposition of iron oxides from produced oil field waters. More particularly, the present invention relates to the use of water soluble polymeric compounds to keep iron oxide particles in produced water suspended.
Oil field produced water is generally saline and often contains significant amounts of dissolved solids, suspended debris from corrosion or the oil-bearing formation, and soluble metals such as iron. Commonly employed control chemicals such as phosphonates, phosphate esters, and a variety of low molecular weight vinyl polymers can help present massive deposition due to the scales such as calcium carbonate, calcium phosphate and calcium sulfate. The dispersing capability of some polymers can keep small particulate matter from settling in lines, tanks and other flow areas. Iron, which normally enters the produced water systems as the soluble Fe.sup.2+ species, can precipitate as the iron (hydr)oxide (a mixture of iron oxides and iron hydroxides). This precipitation, if allowed to go unchecked, can result in flow restriction or line blockage. Ultimately, concentration cells can develop under these deposits and generalized or pitting corrosive attack occurs.
Various systems have been developed in other scientific fields to control or stabilize iron (hydr)oxide in aqueous systems. Such a system is disclosed in U.S. Pat. No. 4,869,828 assigned to Nalco Chemical Company. That system involves treating the aqueous system with from 1.0 to 50 ppm of a water soluble polymer having a molecular weight of from 5,000 to 100,000. The polymer contains from 5 to 85 mole percent pendant N-substituted amide functionality units and from 15 to 95 mole percent of acrylic acid units or a mixture of acrylic acid and (meth)acrylamide units. The N-substituted amide functionality includes amide substituted with sulfoalkyl.
As iron precipitates, it is believed that most of such precipitate is composed of ferric oxide and ferric hydroxide, wherein the iron is in its oxidation state of three, referred to as "iron (III)". For purposes of the present invention, the relative proportions of the precipitate between ferric oxide and ferric hydroxide is not important nor does it matter whether some, or even all, of the iron precipitate is ferrous oxide or ferrous hydroxide. Hence, the iron precipitate which is generally a mixture of ferric oxide and ferric hydroxide, possibly with some ferrous species therein, will be referred to hereinafter as iron (hydr)oxide. It is also to be understood that the term iron (hydr)oxide includes any and all hydrated forms of such oxides and hydroxides.
As petroleum products are produced from fields having high concentrations of iron in the produced water, it is important that a suitable process be developed for controlling deposition of iron precipitates. Additionally, if these fields are located in arctic areas, it would be important to develop a process in which the additive could withstand low temperatures. Such a process is disclosed and claimed herein.